property_descriptor

Properties & Descriptors

Control attribute access with validation, computed fields, and the descriptor protocol.

Files

File	Description
example.py	@property, custom descriptors, validation

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Descriptive Example

Scenario

A Circle class where radius is validated on set and area is computed automatically.

class Circle:
    def __init__(self, radius):
        self.radius = radius

    @property
    def radius(self):
        return self._radius

    @radius.setter
    def radius(self, value):
        if value <= 0:
            raise ValueError("Radius must be positive")
        self._radius = value

    @property
    def area(self):
        return 3.14159 * self._radius ** 2

c = Circle(5)
print(c.area)       # 78.54
c.radius = 10
print(c.area)       # 314.16
c.radius = -1       # ValueError

Custom descriptor

class ValidatedAttribute:
    def __set_name__(self, owner, name):
        self.name = name

    def __get__(self, obj, objtype=None):
        return obj.__dict__.get(self.name)

    def __set__(self, obj, value):
        if value <= 0:
            raise ValueError("Must be positive")
        obj.__dict__[self.name] = value

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Interview Q&A

Q1: What is @property?
A: Decorator that turns a method into a managed attribute. Supports getter, setter, and deleter. Callers use obj.x syntax, not obj.x().

Q2: How does @property work internally?
A: It returns a descriptor object implementing __get__, __set__, __delete__. Python invokes these on attribute access.

Q3: Property vs public attribute?
A: Property adds validation/computation without changing the public API. You can start with public attr and migrate to property without breaking callers.

Q4: What is the descriptor protocol?
A: Object defining __get__, __set__, or __delete__. Class-level descriptors are invoked automatically on instance attribute access.

Q5: What is __set_name__?
A: Called when descriptor is assigned to a class attribute (Python 3.6+). Lets descriptor know its attribute name automatically.

Q6: Real-world descriptor examples?
A: @property, ORM field types (Django/SQLAlchemy), validated attributes, lazy-loaded attributes, bound methods.

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Run

python3 example.py